Agricultural lands are typically comprised of several different soil types, each of which may be categorized according to the relative proportions of sand, clay and silt it contains. A typical agricultural field is usually fertilized with more than one blend of fertilizer, wherein the different soils and/or soil types absorb, release and otherwise react with the various fertilizer blends at differing rates. These differing rates depend upon the soil type, and upon the nutrient components present in the specific types of fertilizers. The three most common nutrient components present in fertilizer blends comprise a nitrogen, phosphorous, and potash. During a typical application of nitrogen, for instance, applying 50 pounds per acre will increase the field nitrogen level by 2%. However, this level of nitrogen can vary depending on the soil type and upon the application of a second and different blend of fertilizer. For instance, if applying 50 pounds per acre of phosphorous would increase the field phosphorous level by 2%, then the level of previously applied nitrogen may have a modified level due to the subsequent application of phosphorous the characteristics of the particular soil type, and the resultant interaction of the nutrient components. Thus, the calculations become rather complex when more than one type of fertilizer blend is added to a particular soil type. Further, since there may be several different types of soil encountered during the application of the fertilizer blends, the calculations can become even more complex.
Studies have been undertaken to quantify the effects of various different fertilizer blends applied in various combinations to different types of soil. Thus, a rather detailed data table or chart is available accounting for the various possibilities of different fertilizer blends combined and applied to different types of soil. Further, effects of sun, wind, and water drainage can also be considered to characterize the fertilizer levels over time based on the topography of the field.
Due to the unique soil distribution and topography of each field, a farmer's task becomes difficult when trying to custom apply a certain quantity of a certain fertilizer to certain areas of the field. Given the desirability of utilizing a large dispensing apparatus to a tractor 11, the various soil conditions and topography encountered during short periods of use can change quickly, since they are not uniformly patterned. Thus, it is not possible or practical for the farmer using conventional apparatus to attempt to reactively alter the dispensing rate of a particular fertilizer or nutrient component in a fertilizer blend as a tractor 11 traverses the field. Rather, with conventional apparatus, a farmer typically averages soil characteristics of a given portion of the field and applies a predetermined quantity of each fertilizer upon that portion of the field. Thus, while the predetermined quantity of a known blend of fertilizer components may be ideally suited for some portions of the land, the certain percentage of one nutrient component in of a blend fertilizer may be too high for some portions while a certain percentage of another is too low for another portion of land.
U.S. Pat. No. 4,630,773 to Ortlip teaches a method and apparatus for spreading fertilizer including a single digital map of the various soil types in a field to be fertilized. This map is normally ascertained from an aerial infrared photograph such that various portions of the land are categorized. The apparatus includes pixels arranged along an x-y locator and wherein the apparatus dispenses a certain percentage of a blend of fertilizers depending upon the location of the dispensing apparatus in the field according to the digital soil map. While this apparatus accounts for the changes of soil types throughout the field, it may not to account for the interactive effects of multiple fertilizers applied to different soil types. Further, the Ortlip apparatus does not consider the effects of fertilizer levels upon a field having varying characteristics due to a varying topography and existing fertilizer levels.